Sheet transport device, image reading device, and image forming apparatus

ABSTRACT

A sheet transport device includes a transport unit that transports a sheet to a reading position by a first transport roller, a driving unit that switches a rotating direction, a rotating unit that rotates the first transport roller in one direction, the rotating unit including a one-direction gear that directly meshes with a first planet gear when the driving unit rotates in a first direction and meshes with a second planet gear when the driving unit rotates in a second direction, a switch unit that switches between states in which rotation of the one-direction gear is or is not transmitted to the first transport roller, and a displacement-force applying part that applies a force for displacing the first planet gear to mesh with the one-direction gear when the first planet gear touches the switch unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-143346 filed Jul. 17, 2015.

BACKGROUND Technical Field

The present invention relates to a sheet transport device, an image reading device, and an image forming apparatus.

SUMMARY

According to an aspect of the invention, there is provided a sheet transport device including a transport unit that transports a sheet to a reading position by a first transport roller and transports the sheet to the first transport roller by switching back and inverting the sheet by a second transport roller after the sheet passes through the reading position, a single driving unit that switches a rotating direction thereof between first and second directions, a rotating unit that rotates the first transport roller in one direction regardless of the rotating direction of the driving unit, the rotating unit including a sun gear driven by the driving unit, first and second planet gears meshed with the sun gear, and a one-direction gear that directly meshes with the first planet gear when the driving unit rotates in the first direction and meshes with the second planet gear with an intermediate gear being disposed therebetween when the driving unit rotates in the second direction, a switch unit that switches between states in which rotation of the one-direction gear is transmitted and not transmitted to the first transport roller, the switch unit being disposed in one axial end portion of the one-direction gear and having a cylindrical exterior, and a displacement-force applying part provided in the first planet gear or the switch unit to apply a force for displacing the first planet gear in a direction to mesh with the one-direction gear when an outer peripheral end of the first planet gear touches the switch unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:

FIG. 1 is an overall configuration view of an image forming apparatus to which an image reading device according to a first exemplary embodiment of the present invention is applied;

FIG. 2 is a structural view of the image reading device according to the first exemplary embodiment;

FIG. 3 is a structural view of a sheet transport device according to the first exemplary embodiment;

FIG. 4 is a structural perspective view of the sheet transport device of the first exemplary embodiment;

FIG. 5 is a structural front view of the sheet transport device of the first exemplary embodiment;

FIG. 6 is a structural plan view of the sheet transport device of the first exemplary embodiment;

FIG. 7 is a structural perspective view illustrating the principal part of the sheet transport device of the first exemplary embodiment;

FIG. 8 is a cross-sectional structural view of an electromagnetic clutch;

FIG. 9 is a structural perspective view of an output roller;

FIG. 10 is a structural perspective view of a tooth-lacking gear;

FIG. 11A is a structural view illustrating a state in which one engaging portion of the tooth-lacking gear is engaged with a distal end portion of a solenoid, and FIG. 11B is a structural view illustrating a state in which the engaging portion of the tooth-lacking gear is disengaged from the distal end portion of the solenoid;

FIG. 12A is a structural view illustrating an intermediate state in which a cam portion of the tooth-lacking gear turns, and FIG. 12B is a structural view illustrating a state in which the cam portion of the tooth-lacking gear is stopped;

FIG. 13A is a structural perspective view illustrating a state in which an end portion of a first planet gear is in contact with a first electromagnetic clutch, and FIG. 13B is a structural perspective view illustrating a state in which the first planet gear is displaced in a direction to mesh with a one-direction gear;

FIG. 14 is a structural perspective view illustrating the principal part of a sheet transport device of the related art;

FIG. 15 is an explanatory view showing the function of the sheet transport device of the first exemplary embodiment;

FIGS. 16-1 to 16-8 are explanatory views illustrating the action of the sheet transport device of the first exemplary embodiment; and

FIG. 17 is a structural perspective view illustrating the principal part of a sheet transport device according to a second exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be described below with reference to the drawings.

First Exemplary Embodiment

FIG. 1 is an overall configuration view illustrating the outline of an image forming apparatus to which a sheet transport device and an image reading device according to a first exemplary embodiment of the present invention are applied.

Overall Configuration of Image Forming Apparatus

An image forming apparatus 1 according to the first exemplary embodiment is configurated as a color copying machine as an example. The image forming apparatus 1 includes an image reading device 3 that reads an image on a document 6 serving as an example of a sheet, and an image forming section 2 serving as an example of an image forming part that forms an image on a recording medium, for example, on the basis of image data read by the image reading device 3 and image data transmitted from an external personal computer. The image reading device 3 is disposed above an apparatus body 1 a that houses the image forming section 2 while being supported by a support section 4. Between the image reading device 3 and the apparatus body 1 a, a space is provided so that a recording medium on which an image is formed is output therethrough.

As illustrated in FIG. 1, the image reading device 3 is provided with a control panel 66 serving as an operating unit used to operate the image forming apparatus 1 and the image reading device 3. The control panel 66 is provided in an upper part of a front wall 311 located on the front of a housing 31 of the image reading device 3. The control panel 66 includes a touch panel 67 and plural operation buttons 68. The touch panel 67 also functions as a display unit for displaying an operation menu, a warning, a message, and so on to the user, and receives various settings for the displayed operation menu.

The image forming section 2 includes plural image forming devices 10 each of which forms a toner image developed with toner contained in developer, an intermediate transfer device 20 that holds toner images formed by the image forming devices 10 and transports the toner images to a second transfer position where the toner images are to be finally second-transferred onto recording paper 5 serving as an example of a recording medium, a paper feeding device 50 that stores and transports required recording paper 5 to be supplied to the second transfer position in the intermediate transfer device 20, and a fixing device 40 that fixes the toner images second-transferred on the recording paper 5 in the intermediate transfer device 20. The apparatus body 1 a is composed of a support structural member, an exterior covering, and so on. The image forming section 2 is not limited to the electrophotographic system, but may adopt another system, such as an inkjet system, a thermal head system, or a lithographic system, as long as it can form images on recording media.

The paper feeding device 50 is disposed to be located below an exposure device 13. The paper feeding device 50 principally includes plural (or singular) sheet containers 51 (51 ₁ to 51 ₄) that contain recording paper 5 of a desired size, type, and so on, and feeding devices 52 and 53 that feed out the recording paper 5 one by one from the sheet containers 51 ₁ to 51 ₄. For example, the sheet containers 51 are mounted to be drawn out toward the front side of the apparatus body 1 a (a side surface the user faces during operation).

Between the paper feeding device 50 and the intermediate transfer device 20, a paper feeding and transporting path 56 is provided to transport recording paper 5 fed from the paper feeding device 50 to the second transfer position. The paper feeding and transporting path 56 includes plural paper transport rollers 54 and 55 and unillustrated transport guides. The paper transport roller 55 disposed at a position just before the second transfer position in the paper feeding and transporting path 56 is configurated, for example, as a roller for adjusting the transport time of the recording paper 5 (registration roller). On the downstream side of the fixing device 40 in the sheet transport direction, a transport roller 58 and an output roller 59 are disposed to output the recording paper 5 to an output container 57 provided in the upper part of the apparatus body 1 a.

The plural image forming devices 10 are formed by four image forming devices 10Y, 10M, 10C, and 10K that form toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Each of the image forming devices 10 (Y, M, C, and K) includes a rotary photoconductor drum 11 serving as an example of an image carrier, a charging device 12 that charges a peripheral surface (image bearing surface) of the photoconductor drum 11, on which an image can be formed, with a required potential, an exposure device 13 that forms an electrostatic latent image (for the corresponding color) having a potential difference by radiating light LB based on image information (signals) onto the charged peripheral surface of the photoconductor drum 11, a developing device 14 that develops the electrostatic latent image with toner of developer of the corresponding color (Y, M, C, or K) into a toner image, a first transfer device 15 that transfers the toner image onto the intermediate transfer device 20, and a drum cleaning device 16 that cleans the photoconductor drum 11 by removing attached substances, such as toner, remaining on the image bearing surface of the photoconductor drum 11 after first transfer.

The intermediate transfer device 20 includes an intermediate transfer belt 21, plural transport rollers 22 to 26 that transport the intermediate transfer belt 21 in a direction of arrow, and a belt cleaning device 27 that cleans a surface of the intermediate transfer belt 21 by removing attached substances, such as toner, remaining on and attached to the surface of the intermediate transfer belt 21. A second transfer roller 28 serving as a second transfer device for second-transferring toner images on the intermediate transfer belt 21 together onto recording paper 5 is disposed in contact with the transport roller 26 with the intermediate transfer belt 21 being disposed therebetween.

The fixing device 40 includes a heating rotating body 41 and a pressurizing rotating body 42. A contact portion between the heating rotating body 41 and the pressurizing rotating body 42 forms a fixing portion that fixes the toner images on the recording paper 5.

The image forming apparatus 1 further includes a duplex unit 60 that forms images on both surfaces of recording paper 5. When recording paper 5 having an image formed on one surface is transported to the output container 57 by the output roller 59, it is introduced into the duplex unit 60 through a switch gate 61 by rotating the output roller 59 in an opposite direction while the output roller 59 holds a trailing edge of the recording paper 5. The duplex unit 60 includes a duplex transport path 65 composed of plural transport rollers 62 to 64 for transporting the introduced recording paper 5 in an inverted state, and unillustrated transport guides.

In FIG. 1, each toner cartridge 140 (Y, M, C, or K) serves as a developer container that stores developer including at least toner to be supplied to the corresponding developing device 14. In the first exemplary embodiment, only toner is stored inside the toner cartridge 140.

A control device 100 controls an image forming operation of the image forming apparatus 1 and an image reading operation of the image reading device 3. A control device for controlling the image reading operation of the image reading device 3 may be provided separately from a control device for the image forming apparatus 1.

Basic Operation of Image Forming Apparatus

The basic image forming operation of the image forming apparatus 1 will be described below.

Here, a description will be given of an image forming operation of forming a full-color image by combining toner images of four colors (Y, M, C, and K) using the four image forming devices 10 (Y, M, C, and K).

When the image forming apparatus 1 receives command information about a request for an image forming operation (printing), the four image forming devices 10 (Y, M, C, and K), the intermediate transfer device 20, the second transfer roller 28, the fixing device 40, and so on start.

In the image forming devices 10 (Y, M, C, and K), toner images of four colors (Y, M, C, and K) are visualized by being developed with the corresponding color toners, for example, on the basis of image data on the surface (first surface) of the document 6 read by the image reading device 3. When the color toner images formed in the image forming devices 10 (Y, M, C, and K) are transported to the first transfer positions, the first transfer devices 15 first-transfer and superimpose the color toner images in order on the intermediate transfer belt 21 that rotates in the direction of the arrow in the intermediate transfer device 20. Next, in the intermediate transfer device 20, the first-transferred toner images are held and transported to the second transfer position by the rotation of the intermediate transfer belt 21. On the other hand, in the paper feeding device 50, required recording paper 5 is fed out to the paper feeding and transporting path 56 in response to the image forming operation. In the paper feeding and transporting path 56, the paper transport roller 55 serving as the registration roller feeds and supplies the recording paper 5 to the second transfer position in accordance with the transfer time.

At the second transfer position, the second transfer roller 28 second-transfers the toner images on the intermediate transfer belt 21 together onto the recording paper 5. In the intermediate transfer device 20 after the second transfer is finished, the belt cleaning device 27 cleans the surface of the intermediate transfer belt 21 by removing attached substances, such as toner, remaining on the surface of the intermediate transfer belt 21 after the second transfer.

Next, the recording paper 5 on which the toner images are second-transferred is separated from the intermediate transfer belt 21 and the second transfer roller 28, and is then transported to the fixing device 40. In the fixing device 40, the unfixed toner images are fixed on the recording paper 5 by a necessary fixing operation (heating and pressurization). Finally, after fixing is completed, the recording paper 5 is output to, for example, the output container 57 disposed in the upper part of the apparatus body 1 a by the transport roller 58 and the output roller 59.

In a case in which images are formed on both surfaces of recording paper 5, when the recording paper 5 is transported to the output container 57 by the output roller 59 after an image is formed on one surface of the recording paper 5, the rotating direction of the output roller 59 is switched to the opposite direction while the output roller 59 holds a trailing edge of the recording paper 5. The transport direction of the recording paper 5 transported in the opposite direction by the output roller 59 is switched toward the duplex unit 60 by the switch gate 61. After that, the recording paper 5 is transported to the paper transport roller 55 in an inverted state through the duplex transport path 65 including the transport rollers 62 to 64. The paper transport roller 55 sends and supplies the recording paper 5 to the second transfer position in accordance with the transfer time. Then, a toner image is second-transferred from the intermediate transfer belt 21 onto a back surface (second surface) of the recording paper 5, for example, on the basis of image data on the back surface (second surface) of the document 6 read by the image reading device 3. The recording paper 5 on which the toner image is second-transferred is subjected to fixing in the fixing device 40. The recording paper 5 is then output with the second surface facing down to the output container 57 disposed in the upper part of the apparatus body 1 a by the output roller 59.

Through the above procedure, the recording paper 5 on which a full-color image is formed by combining toner images of four colors is output. In the image forming apparatus 1, a monochromatic image is formed on recording paper 5 by using only the image forming device 10K for black (K). Structure of Image Reading Device

FIG. 2 schematically illustrates the structure of the image reading device 3 to which the sheet transport device of the first exemplary embodiment is applied.

The image reading device 3 roughly includes a housing 31 having a document reading surface on its upper surface, a document press covering 32 openably and closably attached to the housing 31, and a duplex automatic document feeder (DADF) 33 provided at one end (left end in FIG. 2) of the document press covering 32.

The image reading device 3 is configurated to switch, in response to the operation of the user, the mode of the image reading device 3 between a first reading mode for reading images on front surfaces and back surfaces of documents 6 while automatically transporting the documents 6 one by one by the DADF 33 and a second reading mode for reading an image on a document 6 placed on a document table 83 to be described later. FIG. 2 illustrates states of the members when document reading is performed in the first reading mode.

The DADF 33 includes a document transport mechanism that is composed of a document storage unit 70, a nudger roller 71, a feed roller 72, a pressure pad 72 a, a first transport roller 73, transport rollers 74 and 75, and an output roller 77. The document storage unit 70 can store plural stacked documents 6 while read surfaces (first surfaces) of the documents 6 face up. The nudger roller 71 feeds out the documents 6 from the document storage unit 70. The feed roller 72 loosens and supplies the documents 6 fed out by the nudger roller 71 one by one. The pressure pad 72 a loosens the documents 6 one by one while being in pressure contact with the feed roller 72. The first transport roller 73 transports a fed document 6 to the reading position. The transport rollers 74 and 75 transport the document 6 transported by the first transport roller 73 so that the document 6 passes through the reading position. The output roller 77 serves as an example of a second transport roller that outputs the document 6 to an output storage unit 76. The nudger roller 71, the feed roller 72, the transport rollers 73 to 75, and the output roller 77 are driven by a driving unit (to be described later) during reading of the document 6. The first transport roller 73 is a registration roller that adjusts the transport time of the document 6 to the reading position. Although not explicitly stated in the first exemplary embodiment, the terms “transport roller” and “output roller” include a pair of rollers.

The first transport roller 73 also functions as a correction unit that mechanically corrects the tilt of the document 6 with respect to the transport direction (hereinafter, referred to as “skew correction”) so that the direction of edges of the document 6 becomes the same as the transport direction. As illustrated in FIG. 2, a transport roller 73 b serving as a driving roller in the first transport roller 73 is driven by, for example, a driving unit to be described later, such as a driving motor, so as to rotate in a forward direction Ra in FIG. 2 from a stopped state at a predetermined timing. A transport roller 73 a serving as a driven roller rotates to follow the transport roller 73 b while being in pressure contact with the transport roller 73 b.

In the first transport roller 73, while the transport roller 73 b serving as the driving roller is stopped, a leading edge of the document 6 transported by the feed roller 72 located on the upstream side in the transport direction of the document 6 abuts on a pressure contact portion between the transport roller 73 b and the transport roller 73 a. The first transport roller 73 performs skew correction by starting to transport the document 6 after bending a leading end region of the document 6 so that the leading edge of the document 6 coincides with the axial direction of the first transport roller 73.

The DADF 33 further includes a curved reading guide 78, a back-surface support member 80, a first size detection sensor 81, and a second size detection sensor 82. The reading guide 78 guides the document 6 to the reading position and further guides the document 6 from the reading position in the output direction. The back-surface support member 80 is provided on the reading guide 78 above a reading window 79 to support a back surface of the document 6. The first size detection sensor 81 is provided in the document storage unit 70 to detect the size of the document 6 in the sub-scanning direction. The second size detection sensor 82 is similarly provided in the document storage unit 70 to detect the size of the document 6 in the sub-scanning direction.

The housing 31 of the image reading device 3 is provided as a box shaped like a rectangular parallelepiped whose upper end surface is partly open. The housing 31 includes an upper wall 312 opposed to the document press covering 32, a bottom wall 313 opposed to the upper wall 312, a side wall 314 and a side wall 315 opposed to each other in the sub-scanning direction (a right-left direction in FIG. 2) with the bottom wall 313 being disposed therebetween, the above-described front wall 311 (see FIG. 1), and a rear wall 316 opposed to the front wall 311 in the main scanning direction (a direction orthogonal to the paper plane of FIG. 2).

The upper wall 312 of the housing 31 has a large rectangular aperture 317 in plan view at a position corresponding to the document reading position for the document 6 to be read in the second reading mode. In the aperture 317, a transparent document table 83 (platen glass) is disposed to support the document 6. A portion of the document table 83 on the side of the DADF 33 has a transparent reading window 79 through which the document 6 is read in the first reading mode. Between the reading window 79 and the document table 83, a guide member 84 having an inclined upper end surface is provided to guide the document 6 to the transport roller 75 after the document 6 passes through the reading position in the first reading mode.

The image reading device 3 includes, inside the housing 31, an image reading section composed of a light source 85, a reflector 86, a first mirror 87, a second mirror 88, a third mirror 89, and an imaging lens 91. The light source 85 serves as an illumination unit, such as an illumination lamp or a light emitting diode (LED), which radiates light for illuminating the document 6. The reflector 86 reflects a part of the light emitted from the light source 85 toward a document 6. The first mirror 87 receives reflected light from the document 6. The second mirror 88 receives reflected light from the first mirror 87. The third mirror 89 receives reflected light from the second mirror 88. The imaging lens 91 focuses reflected light from the third mirror 89 onto an image reading element 90 serving as an example of an image reading unit such as a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). The light source 85 radiates light toward the document 6 and the reflector 86. The first to third mirrors 87 to 89 and the imaging lens 91 constitute a reading optical system that reads an image on the document 6 with the image reading element 90.

The light source 85, the reflector 86, and the first mirror 87 are arranged in the main scanning direction, and are mounted in a first moving body 92 formed by a carriage that can be moved in the sub-scanning direction by a driving unit. The first moving body 92 illuminates a reading target region on the document 6 while moving along a first rail 93 in the sub-scanning direction and reflects reflected light from the document 6 toward the second mirror 88 in a second moving body 94 by using the first mirror 87. The first rail 93 is disposed on the rear wall 316 of the housing 31 to extend in the sub-scanning direction.

The second mirror 88 and the third mirror 89 are arranged in the main scanning direction, and are mounted in a second moving body 94 formed by a carriage that can be moved by the driving unit in the sub-scanning direction. The second moving body 94 reflects reflected light from the document 6 toward the imaging lens 91 in the image reading section while moving in the sub-scanning direction along a second rail 95. The second rail 95 is disposed on the bottom wall 313 of the housing 31 to extend in the sub-scanning direction. The single first rail 93 and the single second rail 95 are arranged to be opposed to opposite end portions in the main scanning direction.

The image reading section includes an image reading substrate 97 on which the image reading element 90 is mounted. The imaging lens 91 and the image reading substrate 97 are attached to a base plate 96 supported by the bottom wall 313. In the image reading section, reflected light from the third mirror 89 passes through the imaging lens 91 and is focused on the image reading element 90 such as a CCD or a CMOS, an image on the document 6 is read by the image reading element 90, and image data is output. The image data read by the image reading element 90 is subjected to a predetermined image processing, such as shading correction, by an unillustrated image processing device as required, and is then transmitted to the control device 100.

In the first reading mode, as shown by a solid line in FIG. 2, documents 6 are automatically transported by the DADF 33 in a state in which the first and second moving bodies 92 and 94 are stopped at the reading position set at the left end of the housing 31, a document 6 passing over the reading window 79 is illuminated by the light source 85, and a reflected light image from the document 6 is reflected by the first mirror 87 toward the imaging lens 91 via the second and third mirrors 88 and 89. In the image reading section, the reflected light image from the third mirror 89 is focused on the image reading element 90 by the imaging lens 91, the image on the document 6 is read by the image reading element 90, and image data is output from the image reading element 90.

In contrast, in the second reading mode, the first moving body 92 and the second moving body 94 are driven by the unillustrated driving unit. The moving amount of the second moving body 94 is set to be half of the moving amount of the first moving body 92 so that the optical path length from the image reading position for the document 6 to the image reading element 90 does not change during movement of the first moving body 92 in the sub-scanning direction. In FIG. 2, two-dot chain lines show the positions of the first moving body 92 and the second moving body 94 when the first moving body 92 is moved near an end portion in the sub-scanning direction for the document 6.

Structure of Principal Part of Image Reading Device

As illustrated in FIG. 3, the image reading device 3 according to the first exemplary embodiment includes a sheet transport device 320 that loosens and transports documents 6 serving as an example of a sheet stored in the document storage unit 70 one by one to the reading position and that transports a document 6 having passed through the reading position to the reading position again in a state in which the document 6 is inverted by being switched back and then outputs the document 6. The sheet transport device 320 includes the nudger roller 71, the feed roller 72, the pressure pad 72 a, the first transport roller 73, the transport rollers 74 and 75, the output roller 77, a single driving source 321, a fixed-direction rotation mechanism 322, a first driving transmission switch unit 323, and a second driving transmission switch unit 324. The driving source 321 serves as an example of a driving unit for driving these rollers. The fixed-direction rotation mechanism 322 serves as an example of a rotating unit that rotates the first transport roller 73 in a fixed direction regardless of the rotating direction of the driving motor 321. The first driving transmission switch unit 323 serves as an example of a switch unit that switches between states in which the driving force is transmitted to the first transport roller 73 and is not transmitted thereto. The second driving transmission switch unit 324 serves as an example of a switch unit that switches between states in which the driving force is transmitted to the feed roller 72 and the nudger roller 71 and is not transmitted thereto.

As illustrated in FIG. 3, the sheet transport device 320 includes a normal transport path 325 that transports the document 6 transported by the nudger roller 71 so that the document 6 passes through the feed roller 72, the first transport roller 73, the transport rollers 74 and 75, and the output roller 77, and an inverting transport path 326 that transports the document 6, which is held at a trailing edge by the output roller 77, to the first transport roller 73 again in a state in which the document 6 is inverted by reversing the rotating direction of the output roller 77. The normal transport path 325 includes plural guide members 325 a and 325 b divided along the normal transport path 325 to guide the front and back surfaces of the document 6, the reading guide 78, and the back-surface support member 80. The inverting transport path 326 includes guide members 326 a and 326 b that guide the front and back surfaces of the document 6.

At a branch position where the normal transport path 325 and the inverting transport path 326 separate from each other, a switch gate 327 is provided to switch the transport path of the document 6 from the normal transport path 325 to the inverting transport path 326. For example, the switch gate 327 is formed by an elastically deformable synthetic resin film having a substantially Y-shaped cross section. As illustrated in FIG. 3, the switch gate 327 is usually disposed so that a part thereof obliquely crosses the normal transport path 325. When the document 6 is transported to the output roller 77 along the normal transport path 325, the switch gate 327 is pushed and elastically deformed by the document 6 to open the normal transport path 325 and to switch the transport direction of the document 6 toward the output roller 77. On the other hand, when the document 6 is transported to the inverting transport path 326 by reversing the rotating direction of the output roller 77 while the output roller 77 holds the trailing edge of the document 6, the switch gate 327 switches the transport path of the document 6 transported by the output roller 77 from the normal transport path 325 to the inverting transport path 326.

The sheet transport device 320 includes a document sensor 328 provided at a position corresponding to the nudger roller 71 to detect a leading edge of the document 6. The sheet transport device 320 further includes a pre-registration sensor 329 provided upstream of the first transport roller 73 in the transport direction of the document 6 to detect the leading edge of the document 6.

The structure of the sheet transport device 320 will be described more specifically. As illustrated in FIGS. 4 and 5, the sheet transport device 320 includes a driving motor 321 serving as an example of a driving unit. The driving motor 321 is configurated so that the rotating direction thereof can be switched between a first direction (forward direction) and a second direction (reverse direction). An output gear 332 is attached to an output shaft 331 of the driving motor 321 (see FIG. 5). The output gear 332 rotates in first and second directions R1 and R2 in accordance with the rotating direction of the driving motor 321. The output gear 332 is meshed with a first idler gear 333 a of an integral double idler gear 333. A first driving gear 334 for rotating the first transport roller 73 and the transport rollers 74 and 75 is meshed with a second idler gear 333 b of the double idler gear 333. A sun gear 335 is provided integrally with one axial side of the first driving gear 334, and a driving pulley 336 is provided integrally with the other axial side of the first driving gear 334.

As illustrated in FIG. 6, the first driving gear 334, the sun gear 335, and the driving pulley 336 are rotatably attached to a support shaft 399 laid between first and second frames 397 and 398 of the sheet transport device 320. As illustrated in FIGS. 4 and 5, a driving belt 338 is wound on the driving pulley 336. Further, the driving belt 338 is stretched on a tension pulley 339 that applies tension to the driving belt 338, a driven pulley 341 attached to one end portion of a rotation shaft 340 of the transport roller 74, a driven pulley (not illustrated) that changes the stretching direction of the driving belt 338, and a driven pulley 343 (see FIG. 5) attached to one end portion of a rotation shaft 342 of the transport roller 75. Referring to FIG. 6, a coil spring S applies tension to the tension pulley 339.

As illustrated in FIGS. 5 and 7, first and second planet gears 344 and 345 having a diameter smaller than that of the sun gear 335 are meshed with the outer periphery of the sun gear 335. The first and second planet gears 344 and 345 are rotatably attached to distal end portions of a planet carrier 346 that is rotatably mounted on the support shaft 399 of the sun gear 335. The first and second planet gears 344 and 345 are arranged so that the center angle on the side of a one-direction gear 348 (to be described later) is smaller than 180 degrees. The planet carrier 346 is pressed against a side surface of the sun gear 335 with a required pressure by an annular leaf spring 347 having a curved side surface (see FIG. 4). The planet carrier 346 rotates in the same direction as the rotating direction of the sun gear 335 along with the rotation of the sun gear 335.

As illustrated in FIG. 4, a one-direction gear 348 serving as an example of a one-direction gear part to be selectively meshed with the first or second planet gear 344 or 345 is provided beside the sun gear 335. The one-direction gear 348 is rotatably mounted on a rotation shaft 349 of the first transport roller 73. When the sun gear 335 rotates in the forward direction (clockwise direction in FIG. 4), the one-direction gear 348 is rotated in the clockwise direction in FIG. 4 by being directly meshed with the first planet gear 344 attached to the planet carrier 346. Further, when the sun gear 335 rotates in the reverse direction (counterclockwise direction in FIG. 4), the one-direction gear 348 is similarly rotated in the clockwise direction by being meshed with the second planet gear 345 attached to the planet carrier 346 with an intermediate gear 351 being disposed therebetween. In this way, the one-direction gear 348 constantly rotates in the fixed direction (clockwise direction in FIG. 4), regardless of the rotating direction of the driving motor 321 for driving the sun gear 335 in the forward direction and the reverse direction. The sun gear 335, the first and second planet gears 344 and 345, the intermediate gear 351, and the one-direction gear 348 constitute a fixed-direction rotation mechanism 322.

As illustrated in FIGS. 5 and 7, the intermediate gear 351 is rotatably attached to a distal end 352 a of an arm member 352. The intermediate gear 351 is constantly meshed with the one-direction gear 348. The arm member 352 is attached to the support shaft 399 in a fixed state. As illustrated in FIG. 6, the arm member 352 is disposed between the second frame 398 and the planet carrier 346. As described above, the planet carrier 346 is pressed against the side surface of the sun gear 335 by the annular leaf spring 347 interposed between the planet carrier 346 and the arm member 352. Columnar projections 346 a and 346 b provided on the planet carrier 346 are in contact with side surfaces 352 b and 352 c of a support portion that rotatably supports the first and second planet gears 344 and 345 in the arm member 352. Thus, the planet carrier 346 that rotates together with the sun gear 335 is positioned. The intermediate gear 351 is coaxially provided with a driven pulley (not illustrated) that changes the stretching direction of the driving belt 338. The intermediate gear 351 serves to transmit the rotation to the one-direction gear 348 by reversing the rotating direction of the second planet gear 345. The intermediate gear 351 does not always need to be a single gear, and may be composed of an odd number of intermediate gears meshed with one another.

In the first exemplary embodiment, the same gears are used as the first and second planet gears 344 and 345 and the intermediate gear 351 in order to use parts in common.

As illustrated in FIG. 4, the one-direction gear 348 is attached to a first electromagnetic clutch 323 serving as an example of a first driving transmission switch unit. As illustrated in FIG. 8, the first electromagnetic clutch 323 includes a rotor 323 a, a stator 323 c, an armature 323 d, an excitation coil 323 e, and a cylindrical exterior 323 f. The rotor 323 a is fixed to the rotation shaft 349 of the first transport roller 73 with a D-cut surface 349 a provided on the rotation shaft 349 being disposed therebetween. The stator 323 c is fixed to the rotor 323 a with a bearing 323 b being disposed therebetween. The armature 323 d is rotatably attached to the rotation shaft 349 and is formed by a moving piece to which the one-direction gear 348 is fixed. The excitation coil 323 e is provided in the stator 323 c to form a magnetic circuit that penetrates in the axial direction of the rotor 323 a. The exterior 323 f also functions as a yoke, and protects the excitation coil 323 e. The exterior 323 f is disposed next to the one-direction gear 348. The outer diameter of the exterior 323 f is set to be larger than that of the one-direction gear 348. The exterior 323 f may be provided integrally with the stator 323 c. The stator 323 c is fixed to the second frame 398 of the sheet transport device 320 with a whirl stop portion 323 g being disposed therebetween. The armature 323 d has an unillustrated separating spring that biases the armature 323 d in a direction to separate from the rotor 323 a. This forms a gap G between a vertical surface of the armature 323 d and a side surface of the rotor 323 a.

In the first electromagnetic clutch 323, the armature 323 d and the rotor 323 a are coupled against the separating spring by supplying power to the excitation coil 323 e, and the rotation driving force of the armature 323 d, which is rotated by the one-direction gear 348, is transmitted to the rotor 323 a to rotate the rotation shaft 349 to which the rotor 323 a is fixed.

On the other hand, in the first electromagnetic clutch 323, the armature 323 d is separated from the rotor 323 a by elastic force of the separating spring by stopping the supply of power to the excitation coil 323 e, the rotation driving force of the armature 323 d is not transmitted to the rotor 323 a, and the rotation shaft 349 to which the rotor 323 a is fixed is stopped. At this time, the armature 323 d to which the one-direction gear 348 is attached continues rotation together with the one-direction gear 348.

As illustrated in FIG. 5, a second driving gear 355 for rotating the nudger roller 71, the feed roller 72, and the output roller 77 is meshed with the second idler gear 333 b of the double idler gear 333. The second driving gear 355 is meshed with a driven gear 357 attached to an end portion of a rotation shaft 356 of the output roller 77. Further, as illustrated in FIG. 4, a transmission gear 358 for transmitting the rotation driving force to the nudger roller 71 and the feed roller 72 is meshed with the driven gear 357. To a rotation shaft 359 of the transmission gear 358, a second electromagnetic clutch 324 is attached as an example of a second driving transmission switch unit. The second electromagnetic clutch 324 has a structure similar to that of the first electromagnetic clutch 323. Referring to FIG. 4, the second electromagnetic clutch 324 has a whirl stop portion 324 g.

As illustrated in FIG. 4, a driven gear 360 formed by a helical gear or a spur gear is fixed to the rotation shaft 359 of the transmission gear 358. A transmission gear 361 similarly formed by a helical gear or a spur gear to transmit the rotation driving force to the feed roller 72 is also meshed with the driven gear 360.

In the first exemplary embodiment, as illustrated in FIG. 5, a release unit 362 is provided to release the transport force of the output roller 77 for the document 6 at a preset timing when the rotating direction of the driving motor 321 is switched to the forward direction after the output roller 77 transports the document 6 to the first transport roller 73 by switching back the document 6.

As illustrated in FIG. 3, the release unit 362 is configurated to separate upper driven rollers 77 a, of the output roller 77 composed of a pair of rollers in pressure contact with each other, from lower driving rollers 77 b. As illustrated in FIG. 9, the driven rollers 77 a of the output roller 77 are rotatably attached to distal ends of arms 364 extending from a support shaft 363 rotatably supported by the sheet transport device 32 in an intersecting direction. A lift arm 365 for lifting the driven rollers 77 a in a direction to separate from the driving rollers 77 b is provided integrally with one end portion of the support shaft 363 in a direction intersecting the support shaft 363. As illustrated in FIG. 5, the lift arm 365 is moved up and down by intermittent transmission of driving force from the driven gear 357.

As illustrated in FIGS. 4 and 5, a tooth-lacking gear 367 in which teeth are not provided in a part thereof in the circumferential direction is disposed on one side of the driven gear 357. As illustrated in FIG. 10, the tooth-lacking gear 367 is formed by coaxially combining a first tooth-lacking gear 368 and a second tooth-lacking gear 369. As illustrated in FIG. 5, a solenoid 370 is disposed on one side of the tooth-lacking gear 367 to intermittently drive the tooth-lacking gear 367.

As illustrated in FIG. 10, the first tooth-lacking gear 368 includes a cylindrical large-diameter portion 371 having a relatively large outer diameter, and a small-diameter portion 372 provided integrally with one axial end of the large-diameter portion 371 and shaped like a hollow cylinder having an outer diameter relatively smaller than that of the large-diameter portion 371. On the outer periphery of the large-diameter portion 371, a first gear portion 373 and a second gear portion 374 that form a center angle smaller than 180 degrees are provided at positions symmetrical with respect to the center of the axis. Between the first gear portion 373 and the second gear portion 374, a first tooth-lacking portion 375 and a second tooth-lacking portion 376 are provided so that teeth are not formed therein. On the outer periphery of the small-diameter portion 372, two engaging portions 377 and 378 are provided at positions spaced 180 degrees apart. As illustrated in FIG. 5, the engaging portions 377 and 378 respectively include engaging surfaces 377 a and 378 a provided in the radial direction and inclined surfaces 377 b and 378 b inclined from distal ends of the engaging surfaces 377 a and 378 a to one side in the circumferential direction. When the solenoid 370 is in an off state, a distal end 379 a of an operating lever 379 biased by a spring 370 a engages with the engaging portion 377 or 378 of the small-diameter portion 372, as illustrated in FIG. 5.

In contrast, as illustrated in FIG. 10, the second tooth-lacking gear 369 includes a cylindrical large-diameter portion 380 having a relatively large outer diameter, and a cam portion 381 provided integrally with one axial end of the large-diameter portion 380. Similarly to the first tooth-lacking gear 368, on the outer periphery of the large-diameter portion 380, a third gear portion 382 and a fourth gear portion 383 that form a center angle smaller than 180 degrees are provided at positions symmetrical with respect to the center of the axis. Between the third gear portion 382 and the fourth gear portion 383, a third tooth-lacking portion 384 and a fourth tooth-lacking portion 385 are provided. The third and fourth gear portions 382 and 383 and the third and fourth tooth-lacking portions 384 and 385 are structured similarly to the first and second gear portions 373 and 374 and the first and second tooth-lacking portions 375 and 376, respectively.

The cam portion 381 includes a first flat portion 381 a shaped like a flat surface at a position near the rotation axis, a second flat portion 381 b shaped like a flat surface at a position farther from the rotation axis than the first flat portion 381 a, and curved portions 381 c and 381 d that connect the first and second flat portions 381 a and 381 b.

Between the first tooth-lacking gear 368 and the second tooth-lacking gear 369, a coil spring 386 is provided as an example of a rotation-force applying part that applies rotation force for rotating the first tooth-lacking gear 368 relative to the second tooth-lacking gear 369 in the clockwise direction in FIG. 5. One end of the coil spring 386 is inserted in an insertion hole 387 of the first tooth-lacking gear 368. The other end of the coil spring 386 is inserted in an insertion hole 388 of the second tooth-lacking gear 369 (see FIG. 11).

A pin 389 serving as a guided member is provided on a side surface of the second tooth-lacking gear 369 facing the first tooth-lacking gear 368. On the other hand, a guide groove 390 serving as an example of a guide portion is provided in the first tooth-lacking gear 368 in correspondence with the pin 389. The guide groove 390 is arc-shaped to form a required center angle at a required position around the axis. In a state in which the first tooth-lacking gear 368 and the second tooth-lacking gear 369 are combined, the coil spring 386 applies rotation force for rotating the first tooth-lacking gear 368 relative to the second tooth-lacking gear 369 in the clockwise direction in FIG. 5. In the state of FIG. 5, the pin 389 is located in one end portion 390 a of the guide groove 390.

As illustrated in FIG. 9, a distal end portion 392 of an operating member 391 is disposed to be constantly held in pressure contact with the cam portion 381 of the second tooth-lacking gear 369 by elastic force of an unillustrated coil spring or the like. The operating member 391 is turnably attached to the first frame 397 with a support shaft 393 provided at its base end portion being disposed therebetween. An operating portion 394 for moving the lift arm 365 up and down protrudes from a side surface of the base end portion of the operating member 391 in the axial direction of the support shaft 393. At a distal end of the operating member 391, a planar detector 395 is provided to detect the position of the operating member 391 with a position sensor 396.

As illustrated in FIG. 11A, in the release unit 362, the solenoid 370 is usually in an off state, and the operating lever 379 is engaged with one engaging portion 377 of the tooth-lacking gear 367. At this time, the tooth-lacking gear 367 is stopped at a position where the second and fourth tooth-lacking portions 376 and 385 are opposed to the driven gear 357. For this reason, the rotation driving force of the driven gear 357 is not transmitted to the tooth-lacking gear 367. The first flat portion 381 a of the cam portion 381 in the tooth-lacking gear 367 is in contact with the distal end portion 392 of the operating member 391. Hence, the operating member 391 is located at a position turned in the counterclockwise direction in FIG. 9. For this reason, the operating portion 394 of the operating member 391 is turned downward, the lift arm 365 is pushed down, and the driven rollers 77 a of the output roller 77 are in contact with the driving rollers 77 b.

Next, as illustrated in FIG. 11B, in the release unit 362, when the solenoid 370 is made in an on state only in a preset short time, the engaged state between the distal end 379 a of the operating lever 379 and the engaging portion 377 of the tooth-lacking gear 367 is released. Then, the first tooth-lacking gear 368 of the tooth-lacking gear 367 is rotated by the elastic force of the coil spring 386, and the first gear portion 373 of the first tooth-lacking gear 368 is meshed with the driven gear 357. For this reason, the first tooth-lacking gear 368 is rotated in the counterclockwise direction in FIG. 11B by the driven gear 357. At this time, the second tooth-lacking gear 369 is stopped. When the first tooth-lacking gear 368 rotates a required angle or more in the counterclockwise direction in FIG. 11B, the pin 389 of the second tooth-lacking gear 369 comes into contact with an end portion 390 b of the guide groove 390 in the first tooth-lacking gear 368, as illustrated in FIG. 10. As a result, the second tooth-lacking gear 369 is rotated in the same direction a required angle behind the first tooth-lacking gear 368, and the third gear portion 382 is meshed with the driven gear 357.

When the first tooth-lacking gear 368 rotates to a position where engagement of the first gear portion 373 and the driven gear 357 is finished and the first tooth-lacking portion 375 is opposed to the driven gear 357, as illustrated in FIG. 12A, the rotation driving force is not transmitted from the driven gear 357. However, the elastic force in the clockwise direction in FIG. 5 is applied from the coil spring 386 to the first tooth-lacking gear 368. For this reason, the first tooth-lacking gear 368 continues the rotation in the clockwise direction in FIG. 5, and is stopped in a state in which the other engaging portion 378 is engaged with the distal end 379 a of the operating lever 379 in the solenoid 370, as illustrated in FIG. 12B.

In contrast, as described above, the second tooth-lacking gear 369 rotates in the same direction as the first tooth-lacking gear 368 the required angle behind the first tooth-lacking gear 368. For this reason, at the time when the second tooth-lacking gear 369 rotates to a position where the first tooth-lacking portion 375 of the first tooth-lacking gear 368 is opposed to the driven gear 357, it continues the rotation while the third gear portion 382 is still meshed with the driven gear 357.

After that, as illustrated in FIG. 12B, the second tooth-lacking gear 369 rotates to a position where the engagement between the third gear portion 382 and the driven gear 357 is finished and the third tooth-lacking portion 384 is opposed to the driven gear 357. Then, the rotation driving force is not transmitted from the driven gear 357 to the second tooth-lacking gear 369. At this time, the cam portion 381 of the second tooth-lacking gear 369 is set to be brought into pressure contact with the distal end portion 392 of the operating member 391 between the curved portion 381 d and the second flat portion 382 b after passing through a dead center where the eccentricity amount of the cam portion 381 is the largest. For this reason, at the time when engagement of the third gear portion 382 and the driven gear 357 is finished, rotational moment for rotating the second tooth-lacking gear 369 in the counterclockwise direction in FIG. 12A is applied to the second tooth-lacking gear 369 by the pressing force between the second flat portion 382 b of the cam portion 381 and the distal end portion 392 of the operating member 391. As a result, as illustrated in FIG. 12B, the second tooth-lacking gear 369 rotates and stops at a position where the entire surface of the second flat portion 381 b of the cam portion 381 is in pressure contact with the distal end portion 392 of the operating member 391.

At this time, the distal end portion 392 of the operating member 391 is pushed down by the cam portion 381 of the second tooth-lacking gear 369. In FIG. 9, the operating member 391 is at a position where it is rotated in the clockwise direction. For this reason, the operating portion 394 of the operating member 391 turns upward and pushes up the lift arm 365. The support shaft 363 provided with the lift arm 365 rotates in the counterclockwise direction in FIG. 9. Therefore, the driven rollers 77 a of the output roller 77 move to a position separate from the driving rollers 77 b.

In the tooth-lacking gear 367, the first tooth-lacking gear 368 first stops, and the second tooth-lacking gear 369 rotates in the same direction as the first tooth-lacking gear 368 after some delay and stops. In the meantime, the coil spring 386 is tightened by the second tooth-lacking gear 369 that rotates relative to the stopped first tooth-lacking gear 368.

After that, the tooth-lacking gear 367 is rotated 180 degrees by bringing the solenoid 370 into an on state only for a preset time and returns to the state of FIG. 11A, and the driven rollers 77 a of the output roller 77 come into contact with the driving rollers 77 b.

In this way, in the release unit 362, the operating member 391 is turned by turning the solenoid 370 on and off, the driven rollers 77 a of the output roller 77 are separated from the driving rollers 77 b, and the state in which the transport force of the output roller 77 acts on the document 6 is released.

As illustrated in FIGS. 13A and 13B, the first planet gear 344 in the first exemplary embodiment is provided with a displacement-force applying part that applies a force for displacing the first planet gear 344 in a direction to mesh with the one-direction gear 348 when an end portion of the outer periphery in the axial direction of the first planet gear 344 touches the outer periphery of the first electromagnetic clutch 323.

More specifically, in the sheet transport device 320, when the sun gear 335 rotates in the clockwise direction in FIG. 7, the planet carrier 346 in pressure contact with the sun gear 335 turns in the same direction, and the first planet gear 344 rotatably attached to the planet carrier 346 is meshed with the one-direction gear 348. At this time, as illustrated in FIG. 6, when the thickness of the first planet gear 344 is taken as X and the thickness of the sun gear 335 is taken as Y, a gap A (=Y−X) is set between the sun gear 335 and the first planet gear 344. In this case, the thickness X of the first planet gear 344 and the thickness Y of the sun gear 335 have tolerances α and β including attachment errors, respectively. Therefore, the gap A between the sun gear 335 and the first planet gear 344 is set to be more than or equal to the tolerances (α+β) of the first planet gear 344 and the sun gear 335. The attachment position of the one-direction gear 348 also has a tolerance in the axial direction with respect to the second frame 398.

For this reason, when the planet carrier 346 turns in the clockwise direction in FIG. 7 and the first planet gear 344 meshes with the one-direction gear 348, the first planet gear 344 may touch the exterior 323 f of the first electromagnetic clutch 323 along the axial direction of the first planet gear 344 without meshing with the one-direction gear 348, as illustrated in FIG. 14. If the outer peripheral end of the first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323, the rotation driving force is not transmitted from the first planet gear 344 to the one-direction gear 348, and the first transport roller 73 is not rotated. In this case, transport failure of the first transport roller 73 for the document 6 occurs.

Accordingly, in the first exemplary embodiment, as illustrated in FIGS. 13A and 13B, at least axial end portions (both end portions in the illustrated example) of gear tooth tips on the side of the first electromagnetic clutch 323 at an outer peripheral edge 344 a of the first planet gear 344 have chamfered (so-called C-cut) portions 344 b that are cut at an inclination of 45 degrees to serve as an example of a displacement-force applying part. For example, the first planet gear 344 is produced by injection molding using a synthetic resin material such as POM. At this time, a molding die for injection molding of the first planet gear 344 has chamfered portions 344 b formed beforehand at positions corresponding to the both axial end portions of the tooth tips. Thus, the chamfered portions 344 b are formed in the first planet gear 344 simultaneously with injection molding. As described above, it is only necessary that the first planet gear 344 should have the chamfered portions 344 b at least only at the outer peripheral end opposed to the first electromagnetic clutch 323. In the first exemplary embodiment, however, the chamfered portions 344 b are provided at both axial end portions of the first planet gear 344, that is, the chamfered portions 344 b are also provided at the outer peripheral end that is not opposed to the first electromagnetic clutch 323 in order to cope with automatic assembly and the like by eliminating the directivity in the axial direction of the first planet gear 344.

In the first exemplary embodiment, as described above, the same gears are used as the first and second planet gears 344 and 345 and the intermediate gear 351 in order to use parts in common. For this reason, chamfered portions are provided not only in the first planet gear 344 but also in the second planet gear 345 and the intermediate gear 351.

For this reason, even when the first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323 without meshing with the one-direction gear 348, the chamfered portions 344 b of the first planet gear 344 touch the exterior 323 f of the first electromagnetic clutch 323. As a result, in the chamfered portions 344 b of the first planet gear 344, a displacement force F2 equal to a contact force F1 with which the first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323 acts toward the one-direction gear 348, as illustrated in FIG. 15.

Operations of Characteristic Parts of Image Forming Apparatus

In the image reading device 3 of the first exemplary embodiment, images on the front surface and the back surface of the document 6 are read as follows.

That is, in the image reading device 3, as illustrated in FIG. 3, the nudger roller 71, the feed roller 72, and so on are driven by the driving motor 321, and documents 6 stored in the document storage unit 70 are transported one by one. At this time, as illustrated in FIG. 16-1, the first electromagnetic clutch 323 is in an off state, and the first transport roller 73 is stopped. A document 6 transported by the feed roller 72 is corrected for skew by the contact of its leading edge with the stopped first transport roller 73.

At this time, in the image reading device 3, when the driving motor 321 is rotated in the forward direction R1, as illustrated in FIGS. 4 and 5, the first planet gear 344 meshed with the sun gear 335 moves toward the one-direction gear 348 along with the rotation of the driving motor 321. At this time, as illustrated in FIG. 13A, the outer peripheral end 344 a of the first planet gear 344 sometimes touches the exterior 323 f of the first electromagnetic clutch 323, for example, owing to the tolerance of the first planet gear 344. At this time, as illustrated in FIGS. 13A and 13B, the chamfered portions 344 b are provided at the axial ends of the outer peripheral end 344 a of the first planet gear 344. For this reason, even when the outer peripheral end 344 a of the first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323, the displacement force F2 in the direction to mesh with the one-direction gear 348 is applied to the first planet gear 344 by the chamfered portions 344 b provided in the first planet gear 344, as illustrated in FIG. 15. Therefore, even when the outer peripheral end 344 a of the first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323, the first planet gear 344 is displaced in the direction to mesh with the one-direction gear 348, and meshes with the one-direction gear 348.

After that, as illustrated in FIGS. 16-2 and 16-3, when the first electromagnetic clutch 323 is turned on, the rotation driving force of the one-direction gear 348 is transmitted to the rotation shaft 349 of the first transport roller 73, and the first transport roller 73 transports the document 6 to the transport rollers 74 and 75. An image on the first surface of the document 6 is read by the image reading element 90 stopped at the first reading position in the image reading section while it is passing over the reading window 79, as illustrated in FIG. 2. After that, the document 6 is output onto the output storage unit 76 by the output roller 77 in the case of one-sided reading.

When images on both surfaces of the document 6 are to be read by the image reading device 3, as illustrated in FIG. 16-4, the driving motor 321 is rotated in the reverse direction R2 while the output roller 77 holds a trailing edge of the document 6. When the driving motor 321 is rotated in the reverse direction R2, the transport rollers 74 and 75 driven by the driving motor 321 also rotate in the reverse direction R2. On the other hand, while the first transport roller 73 is rotated in one direction by the fixed-direction rotation mechanism 322, it is stopped by turning off the first electromagnetic clutch 323.

As illustrated in FIGS. 16-4 and 16-5, the document 6 transported in the reverse direction by the output roller 77 is transported to the inverting transport path 326 via the switch gate 327 with its front and back surfaces being inverted. By turning on the first electromagnetic clutch 323 after the leading edge of the document 6 reaches the first transport roller 73, the first transport roller 73 is rotated to correct skew of the document 6. The document 6 is transported to the transport roller 74 by the rotated first transport roller 73. At this time, the transport roller 74 is rotated in the reverse direction by the driving motor 321.

Next, as illustrated in FIG. 16-6, the rotating direction of the driving motor 321 is switched to the forward direction R1 before the leading edge of the document 6 reaches the transport roller 74. After being transported to the reading position by the transport roller 74 rotating in the forward direction R1, the document 6 is transported to the output roller 77 via the transport roller 75. At this time, the rotating direction of the output roller 77 is switched to the forward direction R1 (output direction) along with the switch of the rotating direction of the driving motor 321. For this reason, when the leading edge of the document 6 is held by the first transport roller 73 via the inverting transport path 326, the document 6 may be pulled in opposite directions between the first transport roller 73 and the output roller 77 driven in the output direction. Hence, in the first exemplary embodiment, the solenoid 370 of the release unit 362 is turned on simultaneously with switching of the rotating direction of the driving motor 321 from the reverse direction R2 to the forward direction R1 so that the driven rollers 77 a are separated from the driving rollers 77 b in the output roller 77 to release the transport force of the output roller 77 for the document 6. This prevents the document 6 from being pulled between the first transport roller 73 and the output roller 77.

After that, as illustrated in FIGS. 16-7 and 16-8, the first document 6 is output by the output roller 77, and transport of the second document 6 is started.

In this way, in the first exemplary embodiment, even when the outer periphery of the first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323, as illustrated in FIGS. 13A and 13B, the displacement force F2 in the direction to mesh with the one-direction gear 348 acts on the first planet gear 344. Hence, the first planet gear 344 is meshed with the one-direction gear 348, and the document 6 is transported by the driving force reliably transmitted from the driving motor 321 to the one-direction gear 348.

Second Exemplary Embodiment

FIG. 17 is a structural view illustrating the outline of a sheet transport device according to a second exemplary embodiment of the present invention.

In the second exemplary embodiment, as illustrated in FIG. 17, a chamfered portion 323 h serving as an example of a displacement-force applying part is provided in an axial end portion of an exterior 323 f of a first electromagnetic clutch 323 on the side of a one-direction gear 348.

Therefore, in the second exemplary embodiment, even when the outer periphery of a first planet gear 344 touches the exterior 323 f of the first electromagnetic clutch 323, a displacement force in the direction to mesh with the one-direction gear 348 is applied to the first planet gear 344 by the chamfered portion 323 h provided in the axial end portion of the exterior 323 f of the first electromagnetic clutch 323.

The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

What is claimed is:
 1. A sheet transport device comprising: a transport unit that transports a sheet to a reading position by a first transport roller and transports the sheet to the first transport roller by switching back and inverting the sheet by a second transport roller after the sheet passes through the reading position; a single driving unit that switches a rotating direction thereof between first and second directions; a rotating unit that rotates the first transport roller in one direction regardless of the rotating direction of the driving unit, the rotating unit including a sun gear driven by the driving unit, first and second planet gears meshed with the sun gear, and a one-direction gear that directly meshes with the first planet gear when the driving unit rotates in the first direction and meshes with the second planet gear with an intermediate gear being disposed therebetween when the driving unit rotates in the second direction; a switch unit that switches between states in which rotation of the one-direction gear is transmitted and not transmitted to the first transport roller, the switch unit being disposed in one axial end portion of the one-direction gear and having a cylindrical exterior; and a displacement-force applying part provided in the first planet gear or the switch unit to apply a force for displacing the first planet gear in a direction to mesh with the one-direction gear when an outer peripheral end of the first planet gear touches the switch unit.
 2. The sheet transport device according to claim 1, wherein the displacement-force applying part is formed by an inclined surface provided in an axial end portion of an outer periphery of the first planet gear or the switch unit.
 3. The sheet transport device according to claim 1, wherein a leading edge of the sheet transported from an upstream side of the first transport roller abuts on the first transport roller to correct skew of the sheet when the rotation of the one-direction gear is not transmitted by the switch unit and the first transport roller is stopped.
 4. The sheet transport device according to claim 2, wherein a leading edge of the sheet transported from an upstream side of the first transport roller abuts on the first transport roller to correct skew of the sheet when the rotation of the one-direction gear is not transmitted by the switch unit and the first transport roller is stopped.
 5. The sheet transport device according to claim 1, wherein the second transport roller has a release unit that releases a transport force for the sheet when the rotating direction is switched by the driving unit after the second transport roller switches back and transports the sheet to the first transport roller.
 6. The sheet transport device according to claim 2, wherein the second transport roller has a release unit that releases a transport force for the sheet when the rotating direction is switched by the driving unit after the second transport roller switches back and transports the sheet to the first transport roller.
 7. The sheet transport device according to claim 3, wherein the second transport roller has a release unit that releases a transport force for the sheet when the rotating direction is switched by the driving unit after the second transport roller switches back and transports the sheet to the first transport roller.
 8. The sheet transport device according to claim 4, wherein the second transport roller has a release unit that releases a transport force for the sheet when the rotating direction is switched by the driving unit after the second transport roller switches back and transports the sheet to the first transport roller.
 9. An image reading device comprising: the sheet transport device according to claim 1; and an image reading unit that reads an image from a sheet transported by the sheet transport device.
 10. An image forming apparatus comprising: the image reading device according to claim 9; and an image forming part that forms an image read by the image reading device on a recording medium. 